LC III Modem Port

[brown1d]

Hi,

First a little introduction, I have recently acquired and been repairing a Mac LC III. The machine came without a PSU, so obtained a PSU and recapped it. Machine is running 7.5.5, from BlueSCSI device, it does have a memory and VRAM SIMM. Have OpenTransport 1.1.1 on it, with OpenTransport PPP 1.0.1.

The issue is the Modem and Printer ports. On a US Robotics device, I just see the SD light on when plugged in. The Modem does not dial, or seem to respond. I see CTS and DTR lights flicker. If I put an RS232 breakout box in the cable I see data on the RD line as the Mac tries to command the device. Checked the -5v line and can see voltage, also checked Pin 10 on U3 and U0, and they appear to be also have -5v on them. Pin 10 does not look like its lifted off the board at all. Verified work against Bombard circuit diagram.

Weirdly enough I have had it dial a few times when the top was off, and the PSU put to one side, but on reassembly it does not work again. But I do know the Modem cable is fine (it has worked, and verified wiring with multimeter).

Could it possibly be C22? I see there is a couple of threads on thats orientation. Original CAPs on this board have been replaced, and it looks the cap was installed with the band to the + on the silkscreen. If C22 is removed or failed, will the modem/printer function at all, or intermittently? Re-orienting C22, per the threads, does not seem to help; both pads have continuity to ground and -5v supply lines.

Hope someone seen something like this before.

David

 

[Fred1212]

Link to circuit diagram please.

 

[cheesestraws]

Link to circuit diagram please.

Same place as the others?

On a US Robotics device

Have you tried this just with another computer on the end of the serial line?

Weirdly enough I have had it dial a few times when the top was off, and the PSU put to one side, but on reassembly it does not work again

This almost sounds like something mechanical. You might want to have a good look at the solder joints on the bottom of the modem port. But you haven't really done enough troubleshooting here to be able to tell. Important things to work out:

1. Are you able to connect to another computer with a terminal emulator on each with no hardware handshaking?​
2. Do the two ports behave identically?​
3. Can you use LocalTalk?​
4. Is there any physical damage visible around the ports?​

Going from "the modem doesn't work" to "this capacitor may be at fault" is missing several troubleshooting steps in the middle, and if you just start desoldering things at random you're not likely to make much progress.

 

[brown1d]

So lifted one leg of C22 again, and it magically works. So I think the source of the pain is probably a broken C22 Cap, thats pulling the voltage down. This is similar to what others have seen.

1. With C22 cap soldered down no communication to a Modem or Another computer works
2. Yes both ports behave identically, but both components pull -5v from the same Cap
3. Nope
4. Nope, ports themselves are physically fine

 

[Fred1212]

Thanks Cheesestraws